The present invention relates generally to systems and methods for the recovery of subterranean resources and, more particularly, to a slant entry well system and method.
Subterranean deposits of coal contain substantial quantities of entrained methane gas. Limited production and use of methane gas from coal deposits has occurred for many years. Substantial obstacles, however, have frustrated more extensive development and use of methane gas deposits in coal seams. The foremost problem in producing methane gas from coal seams is that while coal seams may extend over large areas of up to several thousand acres, the coal seams are fairly shallow in depth, varying from a few inches to several meters. Thus, while the coal seams are often relatively near the surface, vertical wells drilled into the coal deposits for obtaining methane gas can only drain a fairly small radius around the coal deposits. Further, coal deposits are not amenable to pressure fracturing and other methods often used for increasing methane gas production from rock formations. As a result, once the gas easily drained from a vertical well bore in a coal seam is produced, further production is limited in volume. Additionally, coal seams are often associated with subterranean water, which must be drained from the coal seam in order to produce the methane.
Horizontal drilling patterns have been tried in order to extend the amount of coal seams exposed to a drill bore for gas extraction. Such horizontal drilling techniques, however, require the use of a radiused well bore which presents difficulties in removing the entrained water from the coal seam. The most efficient method for pumping water from a subterranean well, a sucker rod pump, does not work well in horizontal or radiused bores.
As a result of these difficulties in surface production of methane gas from coal deposits, which must be removed from a coal seam prior to mining, subterranean methods have been employed. While the use of subterranean methods allows water to be easily removed from a coal seam and eliminates under-balanced drilling conditions, they can only access a limited amount of the coal seams exposed by current mining operations. Where longwall mining is practiced, for example, underground drilling rigs are used to drill horizontal holes from a panel currently being mined into an adjacent panel that will later be mined. The limitations of underground rigs limits the reach of such horizontal holes and thus the area that can be effectively drained. In addition, the degasification of a next panel during mining of a current panel limits the time for degasification. As a result, many horizontal bores must be drilled to remove the gas in a limited period of time. Furthermore, in conditions of high gas content or migration of gas through a coal seam, mining may need to be halted or delayed until a next panel can be adequately degasified. These production delays add to the expense associated with degasifying a coal seam.
The present invention provides a slant entry well system and method for accessing a subterranean zone from the surface that substantially eliminates or reduces the disadvantages and problems associated with previous systems and methods. In particular, certain embodiments of the present invention provide a slant entry well system and method for efficiently producing and removing entrained methane gas and water from a coal seam without requiring excessive use of radiused or articulated well bores or large surface area in which to conduct drilling operations.
In accordance with one embodiment of the present invention, a system for accessing a subterranean zone from the surface includes an entry well bore extending down from the surface. A plurality of slanted well bores extend from the entry well bore to the subterranean zone. Drainage patterns extend from the slanted well bores into the subterranean zone.
According to another embodiment of the present invention, a method for accessing a subterranean zone from the surface includes forming an entry well bore and forming a plurality of slanted well bores from the entry well bore to the subterranean zone. The method also includes forming drainage patterns from the slanted well bores into the subterranean zone.
In accordance with still another embodiment of the present invention, a method for orienting well bores includes forming an entry well bore from the surface and inserting a guide tube bundle into the entry well bore. The guide tube bundle includes a plurality of guide tubes. The guide tubes are configured longitudinally adjacent to one another and include a first aperture at a first end and a second aperture at a second end. The guide tubes may also be twisted around one another. A method also includes forming a plurality of slanted well bores from the entry well bore through the guide tube bundle to a subterranean zone.
Embodiments of the present invention may provide one or more technical advantages. These technical advantages may include the formation of an entry well bore, a plurality of slanted well bores, and drainage patterns to optimize the area of a subsurface formation which may be drained of gas and liquid resources. This allows for more efficient drilling and production and greatly reduces costs and problems associated with other systems and methods. Another technical advantage includes providing a method for orienting well bores using a guide tube bundle inserted into an entry well bore. The guide tube bundle allows for the simple orientation of the slant well bores in relation to one another and optimizes the production of resources from subterranean zones by optimizing the spacing between the slanted well bores.